A typical solid oxide fuel cell stack includes multiple fuel cells separated by metallic interconnects (IC) which provide both electrical connection between adjacent cells in the stack and channels for delivery and removal of fuel and oxidant. The metallic interconnect is commonly composed of chromium containing alloys which retain its strength and is dimensionally stable at typical solid oxide fuel cell (SOFC) operating conditions, e.g. 700-900 C. However, during operation of the SOFCs chromium in the alloy reacts with oxygen and forms chromia, resulting in degradation of the adjacent SOFCs.
Two of the major degradation mechanisms affecting SOFCs are directly linked to chromia formation of the metallic interconnect component: i) ohmic resistance increase due to the formation of native chromia (i.e., chromium oxide, which can be expressed as Cr2O3) on the interconnect, and ii) chromium poisoning of the SOFC cathode.
The chromium containing alloy forms the native oxide of chromium oxide (Cr2O3) at SOFC operating temperatures (700-900 C) in both air and wet fuel atmospheres. Although Cr2O3 is electrically conductive, the conductivity of this material at SOFC operating temperatures (700-900 C) is relatively low, with values on the order of 0.01 S/cm at 850 C (versus 7.9×104 S/cm for Cr metal). The chromium oxide layer grows in thickness on the surfaces of the interconnect with time and thus the ohmic resistance due to this oxide layer increases with time.
The second degradation mechanism is known as chromium poisoning of the cathode. During fuel cell operation, ambient air (humid air) flows over the air (cathode) side of the interconnect and wet fuel flows over the fuel (anode) side of the interconnect. At SOFC operating temperatures and in the presence of humid air on the cathode side, chromium on the surface of the Cr2O3 layer on the interconnect reacts with water and evaporates in the form of the gaseous species chromium oxide hydroxide, Cr2O2(OH)2. The chromium oxide hydroxide species transports in vapor form from the interconnect surface to the cathode electrode of the fuel cell where it deposits in the solid form as chromia, Cr2O3. The Cr2O3 deposits on and in (e.g., via grain boundary diffusion) the SOFC cathodes and/or reacts with the cathode (e.g. to form a Cr—Mn spinel), resulting in significant performance degradation of the cathode electrode. Typical SOFC cathode materials, such as perovskite materials, (e.g., lanthanum strontium manganate (“LSM”), LSC, LSCF, and LSF) are particularly vulnerable to chromium oxide poisoning.